A vitrectomy apparatus is provided, including a pressure source, a cut valve connected to the pressure source, the cut valve configured to be turned on and off to provide pressure to selectively extend and retract a vitrectomy cutting device, a plurality of sensors provided at a plurality of points between the pressure source and the vitrectomy handpiece, and a controller configured to employ a function correlating a desired cut rate with a pressure source duty cycle and employ a different function when one sensor of the plurality of sensors senses a pressure outside a predetermined pressure range.